Cardiovascular disease continues to be the leading cause of death globally. A better understanding of how cardiovascular physiology is regulated in health and disease is needed to develop better treatment and prevention of such disorders. Cardiac performance is fine-tuned by an extensive neural network on the heart, so-called the intrinsic cardiac nervous system. Although such local neural circuits are critical for cardiovascular homeostasis and are associated with many serious cardiovascular diseases including arrhythmia and heart failure, the organization of the intrinsic cardiac nervous system and the underlying neural mechanisms for cardiac regulation are poorly understood. The current objective is to bring innovative concepts in neuroscience and novel molecular and genetic technologies to this interdisciplinary field to explore the molecular and functional organization of the intrinsic cardiac nervous system. First, the molecular architecture will be determined using massively-parallel single-cell transcriptome analysis. Intrinsic cardiac neuron (ICN) types will be revealed and neuronal identities of different populations will be resolved by comparing to existing single-cell databases from diverse brain or peripheral regions. Neuronal projections for different ICN populations will be examined using a novel strategy involving virus-based anatomical mapping, whole-heart clearing, light-sheet microscopy, and computational analysis. Functional connectivity of ICNs will be further assessed using innovative ex vivo and in vivo calcium imaging of intrinsic cardiac ganglia to understand how ICNs receive, process, integrate, and deliver signals from the heart and extrinsic cardiac neurons. Physiological roles of ICN populations in health and disease will be revealed using cell-type specific neuromodulatory approaches involving optogenetics, chemical genetics, and diphtheria toxin-mediated ablation. Together, studies proposed here will not only provide us with a comprehensive biological understanding of this crucial cardiac neuroaxis but also help reveal the conceptual framework for functional organization of neural microcircuits. A molecular and functional dissection of the 'little brain' on the heart will open up new vistas in this important area of neural control of the cardiovascular system and may bring novel concepts and therapeutic targets into the field of cardiovascular disease intervention and prevention. My long-term career goal is to better understand the body-to-brain interaction, and to develop novel neuronal-based therapeutic strategies for disease intervention.